It is extremely difficult if not impossible to press and sinter large cross-section soft ductile metal parts having surface and internal compounds which are readily reduced by hydrogen from high purity powder e.g., copper, and obtain high electrical conductivity, i.e., above 92% IACS (International Annealed Copper Standard) throughout the part. It is not unusual to attain a high surface conductivity in a part but attaining a high interior conductivity is quite a bit more difficult. This is largely a matter of controlling the density since the conductivity decreases in approximately a linear fashion with an increase in void volume. Clearly no powder part can achieve a high sintered density if the compacting pressure is so low that a multitude of large pores exist in the green or as-pressed part. With less ductile powders it then becomes simply a matter of increasing the pressure to a practical maximum level. The higher the compacting pressure the higher will be the green density and subsequently the sintered density. However, with ductile powders such as copper, a critical pressure is soon reached beyond which further pressure increase results in an expansion, swelling and even blistering upon sintering. This critical pressure will vary some with the characteristics of the metal powder but in all cases it limits the maximum sintered density of metal powders to that internal conductivities of pressed and sintered parts rarely get above 90% IACS.
It is believed that the cause of swelling that occurs in compacts pressed above the critical pressure is due to early cessation of communication between surface and interior pores. If this occurs before all internal gases, whether present on or within the powder particles or generated by reaction of a reducing gas such as hydrogen with reducible oxides (e.g., oxides of copper, iron, nickel), have escaped from the interior of the compact, swelling and blistering, etc., will occur. The premature closure of the exterior pores is abetted by the smaller voids engendered by the higher pressures, the smearing-densification action of the metal particles located on the periphery of the part next to the die wall, and early sintering of the exterior metal particles due to earlier exposure to the heat and clean-up sintering atmosphere. When blisters occur, very much larger voids are formed.
It was believed that the critical pressure could be raised if in some way the outer pores could maintain their communication with the interior structure and the outside world until substantially all gases are eliminated (except for gases which can readily diffuse through the solid metal such as hydrogen through copper) and then all pores would be allowed to sinter to their minimum volume. It was felt that this concept would be best carried out by using a decomposable or volatile salt which if it left any residue would be both small in volume and insoluble in the metal. This salt could be introduced by admixing with the metal powder before pressing or impregnated into the green compact with an appropriate solution of the salt. This latter technique places the salt exactly where it is required but, of course, the amount of salt that can be introduced is limited by its solubility in the liquid vehicle. For the impregnation method to work the pores at the surfaces cannot be smeared shut by the pressing action or the presence of a wall lubricant. Also some interconnected porosity must exist from surface to interior again limiting the maximum pressure but to higher values than hithertofore. Another critical aspect of the concept is the rate at which the sintering temperature is reached. The decomposition of the salt has to be controlled so that it maintains the interconnection of the pores just long enough to allow all gases to escape and then having done its job must in turn depart without interfering with completion of the sintering operation or itself causing swelling or blistering.